The aim of the present study was to explore the associations between insight and medication adherence at index interview and at 1-year follow-up interview in bipolar and schizophrenic outpatients. The Schedule for Assessment of Insight (SAI) and its expanded version (SAI-E) were used to provide a baseline insight score for 65 bipolar subjects and 74 schizophrenic subjects considered to be in remission or to have minimal psychopathology. Medication adherence of subjects was assessed at index interview and at 1-year follow-up interview, and the association between insight and medication adherence was analyzed cross-sectionally and prospectively. The results of the analysis reveal that in bipolar subjects, the index SAI scores for insight into treatment, mental-health status and psychotic experiences, and total SAI-E were positively correlated with medication adherence at both index and 1-year follow-up interviews. However, in schizophrenic subjects, insight into treatment and total SAI-E correlated positively with medication adherence at index interview but not at 1-year follow-up interview. Medication adherence at index interview could predict medication adherence 1 year later in both bipolar and schizophrenic subjects. These results indicate that the predictive value of insight for medication adherence differs between bipolar and schizophrenic patients, and building insight is an important step for establishing medication adherence in bipolar patients.